Second Order Drift Effects on a Floating Construction

Student thesis: Master thesis (including HD thesis)

  • Stefan Arenfeldt Vilsen
4. term, Structural and Civil Engineering, Master (Master Programme)
In this project the aim is to document the presence of the nonlinearity of a simple wave
group, and examine the effects of the nonlinear waves on a moored floating breakwater.
This is done by producing simple wave groups in a wave flume, and examining the
The laboratory tests were carried out in the wave flume at University of Padova in
cooperation with Professor Luca Martinelli. The wave tests did not yield perfect results,
as the exact generation of the desired waves was not possible. Though the desired waves
were not generated, the results were considered to be suitable for the further analysis.
A model of a floating breakwater was installed with mooring system consisting of
crossing chains. The chains were fitted with strain gauges in order to obtain the forces
in the mooring lines during testing. The motions of the model were recorded by an
optical motion detection system. The results of the laboratory testing are analysed in
order to determine the Response Amplitude Operator (ROA) of the floating breakwater,
giving an indication of the response of the model with respect to the energy content of
the waves.
The RAO is found by determining the transfer function from the wave spectrum to the
spectrum of the response for surge motions in the direction of the wave propagation, the
pitch motions, rotations around the length axis of the model and for the forces occurring
in the mooring lines.
The surge motions show indication of a nonlinear response, with a large response
at low frequencies, while the response at the input frequencies of the spectrum for
all tests is close to 1.0. This is in correspondence with what has previously been
documented regarding the response in a grouped sea state, and clearly shows the
nonlinear behaviour of the waves. The pitch motions were found to have a more linear
response, going well along with the fact that the amplitude of the nonlinear waves are
considered small.
The analysis of the forces in the mooring lines shows a nonlinear response as well.
However, the result of the force analysis does not give good results due to the setup of
the mooring system. The weight of the mooring lines is not enough to prevent snapping
forces to occur, giving an unrealistic response of the forces.
The overall analyses of the performed tests shows the presence of a nonlinear response,
of the floating construction. This indicates that it is relevant to consider the grouping of
waves, when designing the mooring systems of floating constructions.
Publication date10 Jun 2013
Number of pages49
ID: 77394865